Why can I not access the custom EnumProperty which I added to my Panel class?
items = (('OANS', 'oans', 'description', 0))

This is a tuple of four. (x) just evaluates to x. If you want a tuple that contains x, you have to write (x, ). Or just use a list. Either of those two should work:

items = (('OANS', 'oans', 'description', 0), )

items = [('OANS', 'oans', 'description', 0)]